Note: Descriptions are shown in the official language in which they were submitted.
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; The present invention relates generally to the field
of electrophotographic copiers, and,more particularly~ to an
electrophotographic copier having a multiple development capa-
bility for improving the quality of images containing extended
solid and half-tone areas. ~
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. In the typical cascade development station of an
electrophotographic copier, the latent electrostatic image to
be developed and a developer mixture are brought together such
, that toner powder can be attracted to and developed on appropriate
areas of the latent image. In such systems, to produce high :~
quality copies it is essential that the latent image to be
/ ~15 developed, an adequate supply of cleveloper, and a sufficient
o. degree of developer agitation be maintained in the development
'~ : station for a sufficient period of time to ensure that sub-
s~ stantially complete development of the entire image can take place.
Although many systems presently available are able to
~ 20 : accomplish this goal in a generally satisfactory manner when the :
`: document to be copied contains ordinary line copy or the like9
they are not full:y acceptable when the image to be developed
contains large solid or half~tone areas such as pictures, graphs
or the like. In such situations, the latent image must receive
especially large amounts of toner to more fully neutralize the
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black image that approaches the quality obtainable by conventional
printing techniques, and most commercial electrostatic systems
are simply not able to accomplish this in an effective manner,
and, as a result, generally provide copies in which sol~d areas
S are of an inferior quality.
Most of the suggestions that have been made to solve
this problem entail the act of keeping the latent image to be
developed in the development station for a longer period of time
than normal such that larger amounts of toner are able to be
deposited on the image. For example, one suggested solution is to
~ increase the size of the development station and, hence, increase
:' the length of time that the latent image takes in passing through
it. This, however, is not a ver~y good solution because, for one
thing, it tends to make the entire copying system larger, bulkier,
i 15 and more expensive.
,.~ Another approach is to leave the size of the developing
station alone, but instead cause the latent image to move through
it at a slower speed. This is also unsatisfactory because it
requires that other operations of the machine be accelerated or
adjusted to maintain high overall operating speed and this makes
the machine more complex and less reliable.
In general, there is no system presently available
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1~ in the marketplace which can provide high quality copies of
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''! those originals which contain large extended sol~d areas in a
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:¦~ and/or efficiency.
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' In accordance with a presently preferred embodiment
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of the invention, the above inadequacies have been significantly
reduced by providing an electrophotographic copying system
having a multiple development capability which may be utilized
in a selective manner to more fully develop those latent images
which contain extended solid areas. More particularly, the
present invention permits the latent image to be directed through
the development station a plurality of times as needed or
desired to ensure that high density black images of pictures,
graphs or the like can be produced.
This mult;ple development capability is attained by
the provision of a development station of novel construction
which avoids any significant image degradation that would
normally be expected to occur as a result of the image passing
~ through the development station more than one time. In the
'. 15 typical system, multiple development would actually tend to damage
the developed (or partially developed) image more than enhance
`, it, because as the image is passecl through the development
station for a second or subsequent time, there would be a
substantial tendency for the carrier beads to physically rub
against and noticeably damage background areas by producing a high
density of black spots thereon, and, also, to cause edges of
the image to become less well defined. This would happen because
in the conventional system, the carrier beads upon which the toner .
,l particles are electrostatically bound are passed through the
development station at a speed substantially in e~cess of the
speed of the moving latent image to ensure that adequate amounts
. of toner will be supplied to all areas of the image and it is this
velocity differential which creates the rubbing or sweeping
activity.
In the present invention, however, the developing
station has been designed such that the develrper and the latent
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electrostatic image can be move~l through it a-t subs-tantially iden-
tical speeds such that there will not be any significant scrubbing
or wiping ac-tion between them, and because there is no significant
scrubbing, there will be no significant image degradation during
second and subsequent development operations.
Reduction in the rate of travel of the developer rela-
tive to the latent image can be accomplished in several ways while
still ensuring that an adequate supply of toner will be available
to all areas of the image and without sacrificing machine speed
for a single development cycle. In the presently most preferred
embodiment, for example, the spacing or gap between the drum that
~` carries the latent electrostatic image and the development plate
or electrode is reduced significantly. This reduced spacing
results in a higher particle density and a higher frequency of
developer collisions in the gap and, hence, in a slower movement
of the developer through the gap. A:Lso, this increased density
will ensure that a large amount of developer be present adjacent
each area of the image to be developed to ensure full development.
In addition, the development electrode is preferably designed with
' 20 a knurled surface in the manner disc:Losed in copènding Canadian
l;~ Application No. 215,953, filed December 13, 1974.
.~ ` Such an electrode design permits a better and more uni-
-~ form spreading of the developer to be effected to further ensure
that sufficient toner will be available to all areas of the image.
2~ In general, the system provided permits multiple
development to be selectively utilized in those situations when
neeaed or desired to completely develop an image such as
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when the original contains exterlded solid areas. When multiple
developmen-t is not needed, such as when the original contains
only line copy or when improved image quality is not impor-tant,
only a single development need be performed without any loss
in copy speed. Yet further features of the invention will be
set out hereinafter.
~; The accompanying drawing schematically illustrates
the development station of an electrophotographic copier in
` accordance with a presently preferred embodiment of the invention.
The FIGVRE illustrates, in somewhat schematic form,
the development area of an electrophotographic copier. Other
conventional portions of the copier such as the imaging station,
the corona charging station, the exposure station and the drum
cleaning station have not been shown in the interest of clarity
and because they do not form a part of the present invention.
n the FIGURE, reference number l identifies the
photoconductive drum upon the surface of which a latent electro-
static image has been formed in the conventional manner.
Reference number 2 identifies the developer supply hopper while
reference number 3 refers to the development plate or electrode
~ which is spaced from the surface of drum 1 to define a gap or
; space 4 therebetween as understood by those skilled in the art
In operation of the system, developer is dispensed
- from developer hopper 2 and cascades downwardly through the gap
4 such that toner will be attracted to the latent electrostatic
i image on the surface of drum 1 being carried through the develop-
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ment station in the direction in(iicated by arrow 5. The developer
thereafter falls to the bottom of the machine where it is usually
:~ replenished with fresh toner and is picked up by conveyer 6
controlled by a motor 10 which carri.es it back to the top of
the machine to be recirculated therethrough.
After the laten-t image has been suitably developed on
` the drum 1, it is then transferred to a suitable transfer belt 8
: of the type described in copending Canadian Patent Application
No. 184,360, filed October 26, 1973, or to some other medium in ~ ;
a manner recognized by those skilled in the art. In particular,
belt 8 is adapted to be carried across the drum in the direction
. indicated by arrow 7 by a sui-table roller system 9, 11 and can
be selectively brought into or out of contact with the drum by
means of a spring roller 12 or the like as will be explained
in greater detail hereinafter. Belt 8 and its associated
structure does not form part of the present invention, and is
illustrated schematically only to help explain the invention.
It should be clearly understood, however, that the present in-
vention could also be employed in those systems wherein an image
' : 20 is transferred directly from the photoconductive drum to paper
,~ or to other mediums using electrostatic transfer.
Development plate 3 functions as a metallic electrode
and is thus electrically connected to a conventional bias source 13
as is well recognized by those skilled in the art. Preferably also, ;
development electrode 3 is provided with a knurled surface of
the type described in the earlier-mentioned copending Canadian
Patent Applicatlon No. 215,953, which assists in distributing
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the develo~er more uniEormly ov~r tlle en~.ire imacJe to ~2e developed.
An impor-tant feature ~E the inven-tion is that the
gap 4 formed between the surface of clrum 1 ancl plate 3 is
desi~ned to be substantially narrower than -those employed in
conventional systems. Specifically, in the presently con-
structed machine, gap 4 has a spacing of appro~ima-tely 175 mils
in region 16, a spacing of about 100 mils in region 17, a
. spacing of about 85 mils in region 18 and a spacing of about
~ 105 mils in region 19. This variable spacing has been proyided
to ensure a relatively smooth flow of developer into and out of -
gap 4 without creating excessive pile-ups while main-taining the
, very narrow spacing desired in the longer central region of the
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`: gap. Other spacinys, both v~riable and uniform may also be employ- ~
2 ed if desired, however, and, as a general rule Eor best results, a -
spacing of from about 50 mils to about 300 mils with central areas
of the yap preferably beiny less than about 100 mils would satisfy ~.~; : the requirements of the present invention and permit multiple :-
development.
The toner mixture itself is of conventional type consist-
' 20 ing of carrier beads having toner powder adhered thereto and having .
-. an average diameter of about 15 mils. Obviously, for different ~-
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: sl~ed beads, somewhat different gap widths might be employed.
With the development station constructed as described .. ~.
above, the electrophotographic copier will, in only a single :
development cycle, produce copies of a quality equal to or . ~
~: better than those obtainable in present systems. ~t the same ~ :~
.` time, it will also permit images to be developed a plurality ;.
of times so that -those images which contain large or extended :.:
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solid areas can be more completely developed than is possible
with conventional systems. The reason why the present invention
is able to do this can perhaps be better understood by briefly
describing the operation of conventional copiers.
Basically, in conventional development stations the
width of gap 4 is kept at a relatively large size, for example,
250 mils. As a result, the carrier beads tend to fall through
the gap quite freely at a velocity which is significantly
higher than the velocity of the drum surface (which is preferably
` 10 about 10 inches/sec. for an approximately 5 inch diameter drum)
e.g. twice as fast or more. This high developer velocity is
provided intentionally and generally believed to be necessary so
~, as to ensure that an adequate supply of fresh toner will be supplied
to all areas of the latent image as it is passing through the
:
development station. Because of this velocity difference between
~, the developer and the drum surface, however, there will be a
substantial amount of physical contact between the carrier beads
and the latent image on the drum in the form of a tangential
rubbing or wiping action of the beads against the image. Because
. 20 of this rubbing action, second or subsequent development cycles
.~~ would cause unacceptably high background density~ and hence make ~ .
multiple development procedures unacceptable. Furthermore, because
gap 4 is relatlvely wide, conventional systems require relatively
large amounts of developer and this tends to compound the problem
~, 25 because with more developer there will be more rubbing. Thus, in
~ these conventional systems, in order to obtain high density black -
. areas of good quality it has been thought to be necessary to
either sacrifice machine speed or machine compactness by utilizing
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one of the techniques described previously.
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These de~iciencies in the prior art, however, have
been overcome in the present 1nvention by making the width of
gap 4 substantially narrower than in the prior art. Because
the gap 4 is quite narrow, the developer falling through it will
be squee~ed together into a higher collision frequency than in
the prior art. For this reason, they will continuously be
contacting and interacting with one another and thus be forced
to fall at a much lower velocity than in the prior art and, in
fact, will fall at a velocity substantially equal to or only
slightly greater than that of the surface of the rotating drum.
Because the carrier beads and the drum surface are moving at
substantially the same velocity, the amount of scrubbing or
wiping between them will be reduced to a minimum and there will
not be any significant image degradation during second or sub-
`~ 15 sequent development cycles and, hence, multiple development
, becomes possible. In fact, it has actually been found that the
quality of even background portions is somewhat improved by the
~ multiple development procedure. This is believed to be because
-~ in later development cycles, any-backgroun~ toner will tend to be
scavenged by the depleted or partially depleted carrier
beads and this is noticeab1e in improving the quality of back-
, ground areas of the image.
It should also be emphasized that notwithstanding the
fact that the developer and the drum surface move through the
development station at substantially the same velocity, there
is no reduction in the degree of development that takes place
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with only a single development cycle over that found in the
prior art. This is because in the present invention, although
the developer is moving slower than ln the prior art, ;t ;s
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more densely packed in the gap and, hence, there is still an
adequate supply of toner available to develop the imaqe. Also,
because the developer is moving slower, there is more
time for the latent electrostatic image to attract powder to it
than in the prior art. Finally, by also providing the knurled
development electrode as described in the above-referred toCanadian
Patent Application Serial No.215,953, there is substantial
assurance that all areas of the image will receive an adequate
supply of toner. For these reasons, when the image to be copied ~;~
contains only line copy or when time is of the essence, a single
development cycle at speeds at least comparable to those found in
the fastest present-day machines will produce a copy as good as
or better than those presently obtainable, and it is only when
the original contains large solid areas, that multiple develop- ~ -
ment need be used in order to significantly improve the quality
of such images. ~ .
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~-: The number of deve10pment cycles aesired without~.
. image transfer to belt 8 can be selected as desired by means of
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control 21 mounted in the machine console. If the image :
contains prima-rily line copy, a single development is generally
all thàt is needed. If, however, there are extended dark ;
areas on the image, additional cycles, as required, can be~ ~ -
provided merely by turning the dial on control 21. Although
essentially as many development cycles as desired can be carried -
out, (the present invention has actually been practiced with
more than ten cycles without image degradation) the image is
usually fully developed after about three cycles and this is ~- .
all that should normally be necessary. After the desired number
of development cycles has been carried out, control 21 will
automatically actuate roller 12 and belt driving rollers 9, 11
to move the belt 8 into contact with the drum for transfer of
the image thereto.
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Thus, with the present invention a development station
for electropho-tographic copiers has been provided which can
selectively be controlled to develop an image as many times as
desired or as needed for ~he particular type of image being
copied. The system is highly efficient because all that is
required is that a single control be adjusted. If there are no
extended dark areas on the image, a single development is all
that is required and the system will work as fast and as effic-
iently as any other. If there are extended dark areas on the
image, it will, of course, take longer to produce conies but
this will be more than compensated for by the better quality
lmage.
ij It should be understood that the above-described
embodiment is a presently preferred embodiment only. For example,
the multiple development capability described above can also be
~, ~ used to more completely develop images on zinc oxide sheets or
the like to be used in plate making operations. In such
applications, the developed image would not be transferred to a
belt or paper as described above, but, instead, would be used
directly for printing. As used herein, the term "electrophoto- ;~ ;
graphic copier" is also intended to include such an application.
.~ Because many additions, alterations and omissions may be made
from the invention without departing from the scope thereof, it
~ should be recognized that it should be limited only insofar as
;~ 25 requlred by the scope of the Following claims.
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